Left 4 Dead is a Co-op single, and Multiplayer game. In order to better understand the design behind L4D, you'll need to think diversely in both fields of Level Design

• A good map is Fun to play on, and Fresh. The best maps can run on a 24/7 server without getting boring even after the second or third reload.
• A good map accommodates all player ranges of skill, and ability. If the player is failing or having a hard time progressing, neither should the map. Always make the map universal to all kinds of Players.
• A good map is designed, not simply constructed. If all you are doing is re-creating the surface appearance of another map, you're highly unlikely to create something worth playing in its own right.

Starting Off

Left 4 Dead takes place in a zombie apocalyptic Setting. While the rest sprawls through various other scenery. That being said, L4D follows a pseudo Counter-Strike: Source type system. The main flow of the game is directed by an overruling AI, called the info_director. This Director controls waves of enemies, and Dynamic Difficulty.

Environment

Your first goal before starting should be to think like a zombie. However without the idea that your goal is to kill the survivors. The game heavily relies more on the actual waves of zombies influence rather then the players. Because of this zombies can reach just about any height, if of course there is a way to climb up to it.

When thinking about the kind of environment you would like to design, consider the kind of situations the player might find themselves in. Because no one has really lived an actual zombie apocalypse, it's up to you to continually playtest through each of your areas to understand natural decisions any survivor may make. The waves of zombies are dynamically controlled in each section of your map, meaning any real area can be dangerous because of this.

Left 4 Dead excepts non-linearity because initially you are designing your map for two parties: The survivors and the Infected.

Aesthetics

Lighting

A large factor in the design for Left 4 Dead maps is the lighting. Lighting and environment cues are the main ways the level designer can guide the player around the map, as well as giving mood and texture to the map. The original Left 4 Dead maps use a lot of classic horror movie lighting techniques, and you will find that your map will look a lot more professional if you hold to these techniques as well.

These techniques include

• Low-angle lighting - Light sources are placed close to the ground so that the shadows are cast upwards.
• Bright vs Dark - Players will naturally be atracted to light areas and deterrerd from dark areas, and the designer can use this to their advantage.
• Stark contrasts - A very bright area surrounded by darkness is a good way of attracting total attention, and sometimes triggering tunnel vision, the high contrasts in light also give the area a more ominous feel.

Prop placement

Remember that the game is based in a post-zombie-apocalyptic world, so the electrictiy would be out and there would not be many well organised areas. A good rule of thumb is that if you can envision people working or living in the environment without a worry, you probably haven't made it derelict enough. Although by the same token, overcrowding the area can be detrimental to focus, and misleading or frustrating to the player.

Left 4 Dead uses a lot of different decals to bring the world to life, including graffiti decals and community notices. These assist with the players suspension of disbelief by providing more evidence that the world has gone crazy. If a wall looks suspiciously bare, a poster or graffiti spray can help it blend into the environment better.